Telephone and other communication cables are subject to damage and wear from environmental and man made causes. Severe weather conditions such as high winds, snow, icing, rain, floods, and lightning can damage exposed cables. Damage can result from nearby construction or vandalism. The ingress of rain or ground water into the cable core or splice closures at damage locations is a major cause of service interruptions (outages). Every effort is therefore made to keep the cable in good repair and water out of the cable structure.
Frequently, damage does not cause an immediate loss of service but results in a slow degradation of the cable system which often ends in failure of the cable and loss of service. Repair must then be carried out on an emergency basis, which is costly both in restoration costs and lost revenues.
Dating back to the first major cable installations, maintenance monitoring systems have been used to provide early warning of cable trouble. This allows the scheduling of maintenance to avoid lost service and costly repair. The earliest systems used air pressure to keep water out of breaches in the cable or splice closures and to detect damage by measuring the air flow rate into a cable section.
Modern telephone cables, including fibre optic cables, are often filled with water blocking compounds to prevent water migration into the cable core. While providing good resistance to water damage, the filling compounds also block or severely restrict air flow thus making air pressure monitoring systems useless. To overcome this limitation and to provide maintenance monitoring on filled telephone cables, all electronic systems were developed such as those described by McNaughton et al in U.S. Pat. No. 4,480,251 and Vokey et al in U.S. Pat. No. 5,077,526.
Fibre optic cables constructed for applications such as inclusion in overhead power transmission static wires or suspended below phase conductors on transmission towers use all dielectric insulating materials. As a result, conventional cable and splice monitoring methods, such as described by the McNaughton et al and Vokey et al patents which require a metallic electrical conductor element can not be applied.
OTDR techniques, which launch light pulses into a fibre and measure reflected energy to determine loss increases, have been used to monitor optical cables and splices. These methods are expensive and require a special moisture detecting fibre bending device in the splice closures in an attempt to monitor for water ingress. Additionally, the splice bending device is not always predictable in its performance and the exact optical distance to each splice must be known precisely. While it would be preferable to use an active detection device at the splice points, a major difficulty exists in that for all dielectric cable there is no practical means to supply electrical power to splice locations to operate such devices.